U.S. patent number 4,940,305 [Application Number 07/227,826] was granted by the patent office on 1990-07-10 for optical switch based on 1.times.2 directional coupler.
This patent grant is currently assigned to The Boeing Company. Invention is credited to Suwat Thaniyavarn.
United States Patent |
4,940,305 |
Thaniyavarn |
July 10, 1990 |
Optical switch based on 1.times.2 directional coupler
Abstract
An optical switch that does not require precise voltage control
to produce the crossover and straight-through states. The switch
comprises an electro-optic substrate in which a pattern of optical
waveguides is formed, the pattern including an input waveguide that
branches to form first and second output waveguides. The output
waveguides pass through a coupling region within which evanescent
field coupling can occur. A first pair of electrodes produces a
first propagation constant difference .DELTA..beta..sub.1 in a
first section of the coupling region, and a second pair of
electrodes produces a second difference .DELTA..beta..sub.2 in a
second section of the coupling region. By forming the first and
second sections to have different lengths, and by causing
.DELTA..beta..sub.1 and .DELTA..beta..sub.2 to have opposite
polarities, an optical swtich is provided having wide voltage
tolerances for the crossover and straight-through states.
Inventors: |
Thaniyavarn; Suwat (Bellevue,
WA) |
Assignee: |
The Boeing Company (Seattle,
WA)
|
Family
ID: |
22854627 |
Appl.
No.: |
07/227,826 |
Filed: |
March 22, 1989 |
Current U.S.
Class: |
385/16 |
Current CPC
Class: |
G02F
1/3132 (20130101); G02F 2201/125 (20130101) |
Current International
Class: |
G02F
1/29 (20060101); G02F 1/313 (20060101); G02B
006/10 () |
Field of
Search: |
;350/96.13,96.14,96.15 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
Kogelnik, H. and Schmidt, R., "Switched Directional Couplers with
Alternating .DELTA..beta.", IEEE Journal of Quantum Electronics,
vol. QE-12, No. 7, pp. 396 (Jul. 1976). .
Thaniyavarn, S., "Cross-talk Characteristics of .DELTA..beta. Phase
Reversal Directional Coupler Switches", Reprint of Proceedings of
SPIE-The International Society for Optical Engineering, vol. 578,
(Sep. 1985). .
Brochure, "OGW Traveling-Wave 2.times.2 Switch with and without
Fiber Optic Pigtails", Crystal Technology, Inc., 2 pages (Apr.
1988). .
Brochure, "OGW 2.times.2 Switch with and without Fiber Optic
Pigtails", Crystal Technology, Inc., 2 pages (Mar. 1988). .
Thaniyavarn, S., "Self-biased 1.times.2 Modified Directional
Coupler Modulator", SPIE, vol. 789, Optical Technology for
Microwave Applications III (1987). .
Yariv, A., "Coupled-Mode Theory for Guided-Wave Optics", IEEE
Journal of Quantum Electronics, vol. QE-9, No. 9, (Sep. 1973).
.
Thaniyavarn, S., "Modulation Diagrams of Coupled-waveguide
Interferometers and Directional Couplers", 8 pages (Aug. 1987).
.
Schmidt, R. V. and Kogelnik, H., "Electro-optically Switched
Coupler with Stepped .DELTA..beta. Reversal using Ti-diffused
LiNbO.sub.3 Waveguides", Applied Physics Letters, vol. 28, No. 9,
(May 1976). .
Papuchon, M., "Imperial B", Friday Morning, 3 pages (Feb. 1986).
.
"Modified 1.times.2 Direction Coupler Waveguide Modulator",
Electronic Letters, (Aug. 1986)..
|
Primary Examiner: Sikes; William L.
Assistant Examiner: Ngo; John
Attorney, Agent or Firm: Christensen, O'Connor, Johnson
& Kindness
Claims
The embodiments of the invention in which an exclusive property or
privilege is claimed are defined as follows:
1. An optical switch, comprising:
an electro-optic substrate;
a pattern of optical waveguides formed in the substrate, the
pattern comprising an input waveguide that branches to form first
and second output waveguides, at least one output waveguide having
a propagation constant that can be varied by an electric field;
the first and second output waveguides both passing through a
coupling region within which the output waveguides are positioned
so as to produce evanescent field coupling therebetween;
means for selectively generating an electric field in a first
section of the coupling region to thereby produce in the first
section a first difference .DELTA..beta..sub.1 between the
propagation constant of the first waveguide and the propagation
constant of the second waveguide:
means for selectively generating an electric field in a second
section of the coupling region to thereby produce in the second
section a second difference .DELTA..beta..sub.2 between the
propagation constant of the first waveguide and the propagation
constant of the second waveguide, the first and second section
having respective lengths that differ from one another,
.DELTA..beta..sub.1 and .DELTA..beta..sub.2 having opposite
polarities from one another; and
whereby an optical input signal introduced into the input waveguide
can be switched to a selected output waveguide by generating the
first and second electric fields.
2. The optical switch of claim 1, wherein the means for generating
an electric field in the first section comprises a pair of
electrodes positioned adjacent to the first and second output
waveguides in the first section, and wherein the means for
generating an electric field in the second section comprises a
second pair of electrodes positioned adjacent to the first and
second output waveguides in the second section.
3. The optical switch of claim 2, further comprising means for
producing a first voltage difference on the first electrode pair,
and means for producing a second voltage difference on the second
electrode pair, the second voltage difference being equal in
magnitude but of opposite polarity from the first voltage
difference.
4. The optical switch of claim 3, wherein the output waveguides in
the coupling region has a coupling length l.sub.c, and wherein the
coupling region has a length L appraoximately equal to 2l
.sub.c.
5. The optical switch of claim 4, wherein the length of the first
section is approximately equal to 0.68L, and wherein the length of
the second section is approximately equal to 0.32L.
6. The optical switch of claim 4, wherein the coupling region
includes third, forth, and fifth sections that do not include
electrodes, the third section being between the first and second
sections, and the forth and fifth sections being on opposite sides
of the first and second sections respectively with respect to the
third section.
7. The optical switch of claim 6, wherein first through fifth
sections have relative lengths of 0.67; 0.305; 0.005; 0.01; and
0.01, respectively.
8. The optical switch of claim 1, wherein the waveguides are formed
and positioned in the coupling region such that generation of
.DELTA..beta..sub.1 at a first polarity and .DELTA..beta..sub.2 at
a second, opposite polarity causes the optical input signal to
appear only in the first output waveguide, and production of
.DELTA..beta..sub.1 at the second polarity and .DELTA..beta..sub.2
at the first polarity causes the optical input signal to appear
only in the second output waveguide.
9. The optical switch of claim 8, wherein the magnitude of
.DELTA..beta..sub.1 is equal to the magnitude of
.DELTA..beta..sub.2.
Description
FIELD OF THE INVENTION
The present invention relates to electro-optic devices, and in
particular to an electro-optic switch.
BACKGROUND OF THE INVENTION
A known type of electro-optic switch is illustrated schematically
in FIG. 1. The switch comprises a pair of optical waveguides 12 and
14 formed in an electro-optic substrate 16 such as lithium niobate.
Waveguides 12 and 14 include respective coupling sections 18 and 20
that lie within coupling region 22 of length L. Within coupling
region 22, the waveguides are close enough to one another to permit
evanescent field coupling between their respective coupling
sections 18 and 20. Electrodes 24 and 26 are positioned over
coupling sections 18 and 20, respectively, and permit control over
the operation of the switch, as described below.
Whthin coupling region 22, an optical signal passing through one
waveguide will be coupled into the other waveguide. Regarding one
end of waveguide 12 as an input port 32 and the opposite ends of
the waveguides as output ports 34 and 36, and optical input signal
I appearing at input port 32 will in general have its power divided
between the waveguides, such that it produces optical output
signals at both output ports. The output signals at output ports 34
and 36 are designated I.sub.0 and I-I.sub.0, respectively. When an
optical input signal at input port 32 appears only at output port
34, the optical switch is said to be in a straight-through state.
When the optical output signal appears only at output port 36, the
switch is said to be in a crossover state. The straight-through and
crossover states are symbolized by circles containing equal signs
and crosses, respectively, as shown in FIG. 1.
Each particular coupling region between a pair of optical
waveguides is characterized by its interaction length L, by a
coupling length l.sub.c, and by the difference or mismatch
.DELTA..beta. between the propagation constants .beta..sub.1 and
.beta..sub.2 of the respective waveguide coupling sections. Assume
that two waveguides have equal propagation constants, such that
.DELTA..beta.=0. When an optical signal enters the coupling region
in a first one of the waveguides, the power of the optical signal
will begin transferring from the first waveguide to the second
waveguide. After traveling a distance equal to the coupling length
l.sub.c, all of the optical power will have transferred to the
second waveguide. Thereafter, optical power will begin transferring
back to the first waveguide, such that after traveling a total
length of 2l.sub.c, all of the optical power will have returned to
the first waveguide.
As a result of the above properties, complete crossover can be
produced, for waveguides that are exactly phase matched
(.DELTA..beta.=0), when the interaction length L is an exact odd
multiple of the coupling length l.sub.c. In effect, coupling length
l.sub.c is the length of a particular coupling region needed to
transfer all of the optical power from one waveguide to the other
waveguide, when the waveguides are phased matched. When
.DELTA..beta. is not equal to zero, then the transfer of power
between the waveguides is more complex, and in general it may be
impossible to achieve complete crossover using only a single pair
of electrodes.
The device shown in FIG. 1 can be made to operate as an optical
switch by applying a voltage to electrodes 24 and 26. Assume that
the interaction length L is equal to l.sub.c, and that the
waveguides are phase matched (.DELTA..beta.=0) in the absence of an
electric field. Under such circumstances, with zero voltage applied
to the electrodes, the switch will be in the crossover state, and
an input signal at input port 32 will appear only at output port
36, as described above. However, if a differential voltage is
applied to electrodes 24 and 26, then the resulting electric field
in the coupling region will modify the index of refraction of the
electro-optic substrate and of coupling sections 18 and 20, and
will whereby produce a phase mismatch .DELTA..beta. between the
propagation constants of the waveguides. As a result, the input
singal will no longer entirely cross over, and will instead be
split between the output ports. By appropriate selection of the
voltage applied to the electrodes, the switch can be put in the
straight-through state, in which all of the input signal appears at
output port 34.
FIG. 2 shows the output signal I.sub.0 at output port 34 as a
function of the normalized propagation constant difference
.DELTA..beta.L/.pi. between the waveguides. The normalized
propagation constant difference is directly proportional to the
applied voltage. When the applied voltage and normalized
propagation constant difference are equal to zero, the switch will
be in the crossover state. However, when the normalized propagation
constant difference is equal to .+-..sqroot.3, the switch will be
in the straight-through state, and all of the optical power will
appear at output port 34 as signal I.sub.0. Thus the appropriate
voltage applied to the electrodes can cause the optical input
signal to be switched to either output port.
A significant problem with the optical switch shown in FIG. 1 is
that very precise control of the voltage applied to the electrodes
is required in order to produce switching with acceptably low
levels of crosstalk. For example, one needs to apply and maintain
voltages that produce a normalized propagation constant difference
of between -0.03 and +0.03 for the crossover state, and between
1.69 and 1.77 for the straight-through state, in order to keep
crosstalk between the two output ports below the 30 dB level. Other
types of prior directional coupler switches, including those with
multiple electrode sections within the coupling region, have
similarly stringent voltage requirements.
SUMMARY OF THE INVENTION
The present invention provides an optical switch in which less
precision is required in the voltage applied to the electrodes to
achieve the crossover and straight-through states. The switch
therefore permits much greater immunity from crosstalk, in
comparison to prior optical switches.
In a preferred embodiment, the optical switch of the present
invention comprises an electro-optic substrate, and a pattern of
optical waveguides formed in the substrate. The pattern comprises
an input waveguide that branches to form first and second ouput
waveguides, at least one output waveguide having a propagation
constant that can be varied by the presence of an electric field.
The first and second output waveguides both pass through a coupling
region within which the output waveguides are positioned so as to
permit evanescent field coupling therebetween. Means are also
provided for selectively generating electric fields in first and
second sections of the coupling region, the first and second
sections having difference lengths.
In the first section, the electric field produces a first
difference .DELTA..beta..sub.1 between the propagation constants of
the first and second waveguides. Similarly, in the second section,
the electric field produces a section difference
.DELTA..beta..sub.2 between the propagation constants of the first
and second waveguides, such that .DELTA..beta..sub.1 and
.DELTA..beta..sub.2 have opposite polarities. As a result of this
arrangement, an optical input signal introduced into the input
waveguide can be switched to either output waveguide by generating
the electric fields. The characteristics of the switch are such
that the switch can remain in a crossover or straight-through
state, with suitably low crosstalk levels, over relatively wide
ranges of electric field strengths.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic plan view of a prior art optical switch.
FIG. 2 is a graph illustrating the switching characteristics of the
switch of FIG. 1.
FIG. 3 is a schematic plan view of an optical switch according to
the present invention.
FIG. 4 is a schematic diagram showing the electrode connections of
the switch of FIG. 3.
FIG. 5 is a graph illustrating the switching characteristics of the
switch of FIG. 3.
FIG. 6 is a modulation diagram of the switch of FIG. 3.
FIG. 7 is a schematic plan view of a second optical switch
according to the present invention.
FIG. 8 is a modulation diagram of the switch of FIG. 7.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 3 illustrates an optical switch constructed in accordance with
the principles of the present invention. The switch includes a
waveguide pattern 50 formed in an electro-optic substrate 52. The
waveguide pattern has the form of a 1.times.2 directional coupler,
and includes input waveguide 52 that branches at junction 54 to
form a pair of output waveguides 56 and 58. Within a coupling
region 64 of length L, the output waveguides are close enough to
one another to permit evanescent field coupling between the output
waveguides. Input waveguide 52 forms input port 70, and output
waveguides 56 and 58 form output ports 72 and 74, respectively.
The optical switch of FIG. 3 includes a pair of first electrodes 80
and 82 and a pair of second electrodes 84 and 86. First electrodes
80 and 82 extend along output waveguides 56 and 58, respectively,
within a first section 90 of length L.sub.1 of coupling region 64.
Second electrodes 84 and 86 extend along output waveguides 56 and
58, respectively, within a second section 92 of length L.sub.2 of
coupling region 64. Lengths L.sub.1 and L.sub.2 differ from one
another. During operation of the switch, the electrode pairs are
maintained at opposite polarities. For example, if electrode 80 is
positive with respect to electrode 82, then electrode 86 is
positive with respect to electrode 84. For convenience, it will
usually be preferred to apply a voltage to electrodes 80 and 82
that is equal in magnitude, and opposite in sign, to the voltage
applied to electrodes 84 and 86, so that only a single voltage
source is required. Such an arrangement is illustrated in FIG. 4,
wherein voltage source 94 comprises a pair of output terminals 96
and 98. Electrodes 80 and 86 are coupled to terminal 96, while
electrodes 82 and 84 are coupled to terminal 98. FIG. 4 illustrates
the polarity pattern when terminal 96 is positive with respect to
terminal 98. The opposite polarity could also be used.
FIG. 5 illustrates the operating characteristics of a perferred
embodiment of the optical switch of FIG. 3. In this embodiment,
equal magnitude but opposite polarity electrode voltages are used
(as in FIG. 4), the total length L of coupling region 64 is equal
to twice the coupling length (2l.sub.c), and lengths L.sub.1 and
L.sub.2 of the first and second coupling sections are equal to 0.68
L and 0.32 L, respectively. It is assumed that the gap between the
first and second electrode pairs is small enough to be ignored.
FIG. 5 shows the magnitude of the output signal I.sub.0 appearing
at output port 72 as a function of the normalized propagation
constant difference .DELTA..beta.L/.pi.. It may be seen that the
graph includes a relatively long crossover region 100, and a
straight-through region 102 of similar length. Within regions 100
and 102, the characteristics of the switch are relatively
insensitive to the normalized propagation constant difference, and
therefore to the applied voltage. In particular, a crosstalk level
of less than 30 dB can be obtained within very wide ranges of
.+-.1.35 to .+-.1.98 for the normalized propagation constant
difference, ranges that are about an order of magnitude larger than
the corresponding ranges for a conventional directional coupler
optical switch. This flat response to the applied voltage within
regions 100 and 102 eliminates the need for precise voltage control
to achieve low crosstalk levels between the crossover and
straight-through states.
FIG. 6 presents a modulation diagram of the switch shown in FIG. 3.
This diagram plots the normalized propagation constant difference
.DELTA..beta.L/.pi. on the horizontal axis, and plots the
normalized coupling region length L/l.sub.c on the vertical axis.
This diagram assumes that equal magnitude but opposite polarity
voltage are applied within sections 90 and 92, and the length of
sections 90 and 92 are 0.68L and 0.32L, respectively. Solid black
represents a crosstalk level of less than 30 dB, the crosshatched
pattern represents region about the crossover state with crosstalk
between -10 and -20 dB, and the horizontal line pattern represents
regions about the straight-through state with crosstalk between -10
and -20 dB. FIG. 5 can be seen to correspond to a horizontal line
in FIG. 6 at a normalized length value of 2.
The benefits of the invention may be realized by using the
1.times.2 directional coupler waveguide structure shown in FIG. 3,
and by means of two or more unequal length sections, such as
sections 90 and 92, that have alternating polarity. For example,
for an optical switch using three sections within coupling region
64, these sections would all have different lengths from one
another, the first and third sections would have the same polarity,
and the second section would have the opposite polarity. This
requirement can be phrased as one of multiple alternating polarity
electrode sections of unequal length. The magnitudes of the voltage
(and therefore of the nomalized .DELTA..beta. values) need not be
the same for the various sections, although the use of equal
voltage magnitudes is generally preferred to simplify the
implementation. For any particular number of sections selected, one
can pick particular section lengths and voltages, and then derive a
modulation diagram, such as the modulation diagram shown in FIG. 6.
Suitable techniques for deriving such modulation diagrams are set
forth in "Crosstalk Characteristics of .DELTA..beta. Phase Reversal
Directional Coupler Switches," S. Thaniyavarn, SPIE Vol. 578
Integrated Optical Circuit Engineering II (1985), and in "Switched
Directional Couplers With Alternating .DELTA..beta.," H. Kogelnik
et al., IEEE Journal of Quantum Electronics, Vol. QEE-12, No. 7,
July 1976. Once the modulation diagram has been constructed,
inspection of the diagram will reveal whether there are any
normalized length values that include regions of low crosstalk that
extend for a significant distance along the horizontal axis.
The example set forth in FIGS. 3-6 ignores the gap between the
adjacent sections 90 and 92 of coupling region 64. FIG. 7 sets
forth a second preferred embodiment, in which the gap lengths are
taken into account. In this embodiment, coupling region 120 of
total length L is viewed as being divided into five coupling
sections 121-125. Coupling sections 122 and 124 include electrodes,
as in the embodiment of FIG. 3, to which opposite polarity voltage
are applied. Coupling section 121, 123, and 125 do not include
electrodes, and the applied voltage and .DELTA..beta. are assumed
to be zero in these regions. FIG. 8 shows the modulation diagram
for the switch shown in FIG. 7, on the assumption that equal
magnitude but opposite polarity voltages are applied to the
electrode pairs, and that coupling sections 121-125 have the
following relative lengths: 0.01; 0.67; 0.005; 0.305; and 0.01. As
can be seen, selection of a normalized length L/l.sub.c of about 2
again produces an effective optical switch having large crossover
and straight-through regions in which low crosstalk is maintained
despite relative large swings in the electrode voltage.
While the preferred embodiment of the invention have been
illustrated and described, variations will be apparent to those
skilled in the art. Accordingly, the invention is not limited to
the specific described embodiments, but is instead to be determined
by reference to the following claims.
* * * * *